Traditional solar thermal energy generation (STEG) systems utilize reflectors, usually a parabolic mirror or system of mirrors, that concentrate the solar radiation at a focal point. At the focal point, water is super-heated to create steam; the super-heated steam is used to power a turbine, configured in a traditional Rankine cycle, and turn a generator to produce electricity. A STEG system that operates in this manner is significantly less energy efficient than fossil fuel or nuclear power plants. These inefficiencies stem from the inability to harvest a constant and predictable amount of solar radiation due to daily, seasonal, and weather-related fluctuations, and the under-utilization of operational facilities during periods of low solar radiation. A major shortcoming of a traditional STEG system lies in the inability to control electricity output to meet predicted electricity needs. This lowers the value proposition for STEG systems, especially in the electricity generation market.
Newer designs for STEG systems employ concentrated solar radiation focused on a housing filled with a heat transfer fluid (HTF). The HTF has the ability to store thermal energy for use during periods of low solar radiation. These designs, however, require the use of dense molten salts or molten alkali metals for the HTF, which must be located at the top of a tall receiving tower. The HTF must then be pumped through a thermal circuit so that the stored thermal energy can be utilized by the generation facility. These technical challenges increase the capital and maintenance costs of HTF based STEG systems. Storing the energy in the HTF only guarantees power generation for a short period of time, usually hours, and cannot provide power during long periods of cloud cover. Furthermore, these systems are still subject to the seasonal fluctuations of solar radiation. These factors make the storage of thermal energy impractical for large scale penetration into the energy market.
Fossil fuels can be used in power generation plants and can be controlled to meet predicted electricity needs; however, the availability of fossil fuels is limited in nature and will eventually be depleted. Moreover, their use is harmful to the environment and to human health, and, dependence on fossil fuels carries political ramifications.
The hybridization of solar and non-solar technologies is one potential solution to these problems, but current designs have drawbacks. Many prior art hybrid solar thermal systems use a HTF with a heat exchanger. However, such an approach often preheats the working fluid (e.g., water) or performs other secondary heating tasks and do not provide energy directly to the heating vessel (e.g., boiler) that heats the working fluid. As a result, only a small portion of the total energy generated is attributable to the use of solar radiation. For example, Bharathan (U.S. Pat. No. 5,417,052), incorporated herein by reference, teaches a hybrid solar central receiver for a combined cycle power plant including a molten salt HTF to preheat air from the compressor of a gas cycle. However, the requirement of a central receiver, molten salts, and a heat exchanger represent a large infrastructure investment. The hybrid solar central receiver also uses solar generated heat only to preheat air for a natural gas turbine in a secondary heating role.
It is desirable to provide a hybrid solar/non-solar energy generation system and corresponding energy generation method that is highly efficient and address these drawbacks of the prior art.